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U. Glaeser

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out[n]<br />

a0 FIGURE 31.11 Transmitter pre-distorted waveform (a) and implementation (b).<br />

the driver more strongly for a period immediately after a data transition so that the transmitter drives<br />

higher frequency components with more signal power. For more complex channel responses, a programmable<br />

filter precedes the actual line driver and inverts the effect of the channel. Figure 31.11(a) illustrates<br />

an example of an analog filter. The length of the optimal filter depends on the tail of the pulse response.<br />

For many cables (less than 10 m) one or two taps is sufficient [6,13,14]. Figure 31.11(b) shows the effect<br />

of transmitter equalization. The small, negative pulses before and after the original pulse eliminates the<br />

tails of the pulse response.<br />

A digital-FIR filter would output a quantized word that represents the output voltage. Instead of<br />

transmitting two levels, the driver is a high-speed D/A converter. Because current or impedance control<br />

uses binary-weighted driver segments, designs for a D/A converter are not significantly different; however,<br />

a design with linearity of >6 bits at multi-GSamples/sec faces challenging issues: device mismatches limit<br />

linearity, transmit clock jitter limits the SNR, and the switching of output transitions induce glitches.<br />

Thermal noise for a 50-Ω environment is approximately 1nV/ Hz<br />

and only limits resolution at very<br />

high resolution. Recent research has demonstrated this potential with

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